Discovery Of Senescing Strains

Since Podospora and Neurospora (Ascomycotina) are commonly used in genetical research, their stocks are maintained by regular subculturing. Not surprisingly, the phenomenon of senescence is best documented in these two fungi. In the mycelium of Podospora anserina, the mycelium grows to a limited extent following the germination of ascospores. Therefore, senescence in this species is a part of its normal development. On the other hand, Neurospora is potentially immortal. Some stocks of this fungus used by Beadle and Tatum over fifty years ago (Chapter 5) are still alive after numerous subcultures. However, nearly 30% of strains of N. intermedia collected from the island of Kauai in Hawaii and from Maddur in peninsular India have died in 5 to 50 subcultures. Initially, a senescent strain is morphologically indistinguishable from a long-living strain. However, as subculturing continues, the quantity of aerial mycelium is reduced, conidia are not formed, the respiratory activity diminishes and levels of cytochromes decrease

(Figure 14.2). Eventually, growth ceases completely and the culture is regarded to have died. Thus, if a new isolate is likely to be used for investigation of senescence phenomenon, parallel subcultures from the same strain should be metabolically immobilized by lyo-philization, storage in anhydrous silica gel or by cryopreservation.